Wednesday’s announcement that the next iteration of the Apple Watch can both monitor the wearer’s heart rhythm and, if a suspicious reading emerges, perform an electrocardiogram, could be a boon for users and their doctors. Or it could be a massive headache for the health care system.

The new watch continuously monitors the wearer’s heart rate. It does this by shining a light onto the back of the wrist. An algorithm analyzes the light that’s reflected back, which changes with each pulse. If a suspicious reading appears, the watch notifies its user, who can then make the watch perform an electrocardiogram. That test offers a more sensitive and accurate picture of the heart’s rhythm.

According to the Food and Drug Administration, which cleared Apple’s monitor and electrocardiogram applications, the continuous monitoring and notification app isn’t for people under age 22 or those diagnosed with atrial fibrillation, an irregular and often rapid heart rate that affects as many as 6 million Americans. Atrial fibrillation causes symptoms such as fatigue and shortness of breath in some, and increases the risk of having a stroke four- or five-fold.

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I’m a cardiologist with a keen interest in heart rhythm problems. I’ve found that devices that do things similar to what Apple says its watch will do to be unbelievably helpful for my patients with atrial fibrillation.

Atrial fibrillation comes and goes. Not long ago, to definitively diagnose it I had to rely on patients experiencing an episode of atrial fibrillation while they were in the exam room, or seeing it on the recording of a cumbersome Holter monitor. That changed when the FDA approved in 2013 a device made by AliveCor called the KardiaMobile EKG. It has revolutionized how I manage patients with atrial fibrillation.

This small $99 device can capture a medical-grade electrocardiogram in about 30 seconds and with reasonable accuracy interprets the EKG as either normal, unclassified, or possibly atrial fibrillation. Once one of my patients takes an EKG and sends me an alert, I can assess his or her heart rhythm in the cloud and see if it is in the normal range or is worrisome. I sometimes joke that I am navigating blindly in the wilderness with patients of mine who don’t have one of these devices when they call and tell me they are having symptoms.

The Apple Watch may do the same thing, so I am eager to try it out. But it will cost a lot more and we don’t know how accurate it is.

I have some reservations about the Apple Watch as a heart rhythm monitor, especially among healthy people. We know a lot about the heart, but it can often play innocuous tricks. Apple claims that its optical sensor algorithms can detect atrial fibrillation, and the watch will warn users when it thinks they are in this dangerous heart rhythm based on what the optical sensor says. That may send a lot of people whose hearts are doing perfectly fine to their doctors for unnecessary and potentially harmful testing.

We also don’t know how accurate the Apple electrocardiograms are. I’m shocked that the FDA cleared this application since I haven’t seen any data on it.

The Apple Watch essentially says “Your heart rate is normal” or “You are having atrial fibrillation.” But most people who have their heart rhythms measured end up as unclassified, somewhere between a normal heart rhythm and atrial fibrillation. General physicians, and even cardiologists, sometimes have trouble sorting out what’s normal and what’s atrial fibrillation. I’d like to know how Apple is planning to handle situations in which its algorithm is confused.

The devil, they say, is in the details. The announcement of a new product is merely a promise for the future. I’ll be keeping an eye on the Apple Watch to see how well it monitors heart rhythms and how accurately it takes electrocardiograms. If it does a great job with these, then it’s a welcome addition to the burgeoning universe of personal health devices. If it doesn’t, and cardiologists and emergency departments are overwhelmed with falsely worrisome heart rhythm readings, we may need to reassess how devices like the Apple Watch are reviewed and cleared.

Anthony Pearson, M.D., is medical director of the echocardiography laboratory at St. Luke’s Hospital in Chesterfield, Mo. He blogs about cardiovascular disease at The Skeptical Cardiologist.

The new watch continuously monitors the wearer’s heart rate. It does this by shining a light onto the back of the wrist.

No, it doesn’t. The Apple Watch 4 has electrodes for the back of the wrist of one arm and a finger of the other arm. It is a true EKG, though a simple one. Previous generations of the Apple Watch did use optical sensors for pulse, and they were much less reliable for detecting atrial fibrillation.